Electrophotography utilizes the formation of an electrostatic latent image to create a hard copy reproduction. In its basic aspects, a laser printing engine 124, shown schematically in FIG. 1 (Prior Art) applies a charge with a scorotron charger 136 to a moving photoconductive insulating surface area of a photoconductor 126. The surface area is exposed to a pattern of light 138, 140. A latent image of the pattern is formed on the charged surface which is then developed by application of electroscopic toner 128, 130, 132, 134 to the photoconductive material. The developed image is transferred to a hard copy medium 152 using a transfer drum 148 with a transfer corona charge unit 150 and fused, or fixed, to the medium 152 by using another transfer corona unit 154. The photoconductive material insulating surface is then erased 146, cleaned 142, 144, and reused for the next image. This basic construct is used in a variety of state of the art products such as computer printers and plotters, copiers, facsimile machines, and the like.
In the field of color hard copy reproduction, such as by laser printers using liquid electrophotography (LEP) techniques, the use of color liquid toners (generally yellow 128, magenta 130, cyan 132 (the subtractive primary colors) and a black toner 134) that are difficult to process presents challenging designs problems. Each printing cycle must charge, expose, develop, and transfer colors, several being through a toner layer that has already been deposited on the photoconductor 126. One problem inherent in the process is the managing of excess liquid toner.
Pneumatic pressure has been used to control excess liquid toner. For example, U.S. Pat. No. 3,741,643 uses an "air knife for removing excess toner from the surface of the photoconductive drum or belt." Col. 1, II. 35-36. Essentially, the forced air is used to evaporate the "diluent" part of the liquid toner.
Referring to FIG. 2 (Prior Art), another solution to the problem of dealing with excess liquid toner has been to add a squeegee roller 229 adjacent to the developer roller 228 of each developer assembly. While effective at drying the photoconductor at the surface, the squeegee roller 229 is known to leave the imaged photoconductor wet with toner at its outside edges (also known as edge effects), that is, along each end of the squeegee roller proximity area with the photoconductor 126 (FIG. 2A). This area of wet photoconductor is drawn into the next different color developer where it mixes with that toner. Over time, this color mixing, known as cross-contamination, is sufficient to seriously degrade color print quality. Various devices such as having absorbent pads, suction devices, or counter-rotating end caps at each end of the squeegee roller have provided limited success at controlling edge effects. Therefore, there is a need for an apparatus to assist squeegee roller to prevent these edge effects that lead to cross-contamination.
Moreover, it is known that a squeegee roller 229 retains a volume of toner across a substantial part of its surface area after wiping an image on the photoconductor 226. As demonstrated in FIG. 2B, a drip line of retained toner forms in the downstream nip between the squeegee roller 229 and the photoconductor 226 as the toned image pulls away from the squeegee roller 229. This volume of retained toner is known to be sufficient to contaminate the colors of the adjacent developers. Over time, the wasted toner from the drip line effect will also seriously reduce the number of pages that can be printed from a given volume of toner. Such drip lines have also been found to form on the developer roller 228. Therefore, there is also a need for an apparatus to alleviate the drip line effect problem.